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A multispectral scanning spectrometer was used to obtain measurements of the bidirectional reflectance and brightness temperature of clouds, sea ice, snow, and tundra surfaces at 50 discrete wavelengths between 0.47 and 14.0 mm. These observations were obtained from the NASA ER-2 aircraft as part of the First ISCCP (International Satellite Cloud Climatology Project) Regional Experiment (FIRE) Arctic Clouds Experiment, conducted over a 1600 km 3 500 km region of the north slope of Alaska and surrounding Beaufort and Chukchi Seas between 18 May and 6 June 1998. Multispectral images in eight distinct bands of the Moderate Resolution Imaging Spectroradiometer (MODIS) Airborne Simulator (MAS) were used to derive a confidence in clear sky (or alternatively the probability of cloud) over five different ecosystems. Based on the results of individual tests run as part of this cloud mask, an algorithm was developed to estimate the phase of the clouds (liquid water, ice, or undetermined phase). Finally, the cloud optical thickness and effective radius were derived for both water and ice clouds that were detected during one flight line on 4 June. This analysis shows that the cloud mask developed for operational use on MODIS, and tested using MAS data in Alaska, is quite capable of distinguishing clouds from bright sea ice surfaces during daytime conditions in the high Arctic. Results of individual tests, however, make it difficult to distinguish ice clouds over snow and sea ice surfaces, so additional tests were added to enhance the confidence in the thermodynamic phase of clouds over the Chukchi Sea. The cloud optical thickness and effective radius retrievals used three distinct bands of the MAS, with a recently developed 1.62- and 2.13-mm-band algorithm being used quite successfully over snow and sea ice surfaces. These results are contrasted with a MODIS-based algorithm that relies on spectral reflectance at 0.87 and 2.13 mm.

The antiferromagnet Ce8Pd24Ga with TN = 3.1 K has been investigated using neutron diffraction, inelastic neutron scattering, electrical resistivity, magnetoresistance and magnetic susceptibility measurements. Rietveld analysis of neutron diffraction data reveals that Ce8Pd24Ga crystallizes in the cubic structure with space group Pmm. Inelastic neutron scattering (INS) studies show two well-defined crystal-field excitations at 3.2 meV and 12.8 meV. The crystal-field parameters have been estimated from the analysis of INS data. The heat capacity calculated from the crystal-field level scheme shows a Schottky peak at 15 K which agrees well with the reported experimental results. The resistivity exhibits -ln T behaviour at high temperature followed by a peak at 8 K and eventually drops at TN. The peak in the resistivity at 8 K arises due to the combined effect of crystalline electric fields and Kondo interactions. The analysis of the resistivity data in the magnetically ordered state reveals a gap of 16.1 K in the spin-wave spectrum. At 1.8 K the magnetoresistance is positive and it changes to negative at 3 K. The positive magnetoresistance at 1.8 K is consistent with the antiferromagnetic ground state. The negative magnetoresistance shows a scaling behaviour that yields a low-temperature Kondo temperature of 5.8 K. The magnetic susceptibility exhibits Curie-Weiss behaviour between 20 K and 300 K with an effective paramagnetic moment µeff = 2.33 µB and paramagnetic Curie temperature θp = -18.8 K. The present studies reveal that the physical properties of Ce8Pd24Ga are governed by the Kondo, crystal-field and Ruderman-Kittel-Kasuya-Yosida (RKKY) interactions.

Neutron scattering, thermal expansion, and magnetostriction measurements have been carried out on the cubic compounds ${\mathrm{CeSbNi}}_{x}$ $(x=0--0.4).$ Inelastic neutron scattering studies show a well-defined crystal field (CF) excitation at 4.06 (\ifmmode\pm\else\textpm\fi{}0.04) meV and 5.02(\ifmmode\pm\else\textpm\fi{}0.05) meV in $x=0.08$ and 0.15, respectively. The crystal field splitting increases with x compared with the parent compound CeSb (3.19 meV) in spite of the lattice parameter increasing with Ni incorporation. The implication is that the increase in the CF splitting in $x=0.08$ and 0.15 is due to a collapse of the p-f mixing between the Sb $5p$ holes and the localized Ce $4f$ electrons. The analysis of inelastic spectra of $x=0.15$ shows that the ground state is a doublet $({\ensuremath{\Gamma}}_{7}),$ which explains the temperature-dependent behavior of the magnetic susceptibility. Thermal expansion shows a dramatic change in behavior with Ni composition. The thermal expansion coefficient exhibits a first-order transition at 15.4 K in CeSb, which disappears for Ni composition as low as $x=0.035,$ as well as in an applied field of 8 T. A large magnetostriction has been observed in CeSb in the magnetic-ordered state as well as in the paramagnetic state. The absolute values of the magnetostriction are reduced considerably in the Ni-incorporated alloys. The volume magnetostriction of $x=0.15$ alloy exhibits a scaling behavior in the paramagnetic state from which we have estimated the product of the magnetovolume coupling constant and the isothermal compressibility.

We describe the design, construction and use of a new swept frequency pulsed magnetic resonance spectrometer that is particularly suitable for measurements of NMR in magnetically ordered materials but has also been used for ESR, NQR and conventional NMR. It includes computer control, digital pulse generation, measurements over the range 10-1000 MHz in one sweep, and calibration of signal and pulse power. The spectrometer can also be used to track the change of the NMR frequency of a sample, e.g., as a function of temperature.

59Co nuclear magnetic resonance (NMR) measurements have been undertaken to determine the local atomic environment of Co in Co/V multilayers, previously studied by magnetometry. These NMR measurements provide confirmation, on a local atomic scale, of the results deduced from the earlier magnetization study. Co/V multilayers show considerable interfacial mixing with a magnetically dead, vanadium rich layer penetrating approximately 2 A into an adjoining Co layer. The presence of this dead layer has the effect of modifying the hyperfine field experienced by the Co nuclei at a distance of several atomic planes from the interface.

The inclusion of a Au layer in the magnetic tunnel junction structure Fe/MgO/CoFe substantially alters the magnetic properties. In this work we use zero field /sup 59/Co NMR to see if these changes in magnetic properties are correlated with changes in the local atomic environment. The technique of spin-echo NMR allows the effective magnetic field (hyperfine field) at the nucleus to be determined and so provides information on the local atomic environments including strain effects and magnetic moments. The results show that the local atomic environment of Co does not change significantly, however the r.f. power required to obtain the maximum signal is substantially different when the Au layer is included showing that the r.f. electronic enhancement factor changes appreciably.

This review continues earlier surveys1–5 of work using High Field EPR (HFEPR) and covers the years 2004 and 2005. Over the last 10 years there has been a rapid expansion of research using HFEPR, conventionally defined as requiring frequencies at and above 90 GHz. In particular the introduction of co...

59Co NMR and magnetotransport measurements have been undertaken on a series of molecular beam epitaxy grown Co 22 A/Cu 32 A(111) multilayers as a function of the number of repeat units in the multilayer stack. Examining the positions of the bulk [Co surrounded by 12 Co nearest neighbors (NN)] and ideal planar interface (Co surrounded by 9×Co and 3×Cu NN) lines showed that increasing the number of repeat units leads to greater strain in the Co layers indicating better epitaxial registry. Interfacial mixing was studied by examining the ratio of the relative signal strength of the bulk and ideal planar interface lines compared to the signal outside these regions. The relative intensity of the NMR signal outside the bulk and ideal interface lines clearly reduces as the number of repeat units is increased. This demonstrates that in these samples the interfacial quality improves as the number of repeat units is increased. Magnetotransport measurements show that the improvement in structural quality could be cor...

59Co nuclear magnetic resonance (NMR) spectra of small Co particles in a SiO2 matrix have been measured. In zero field, strong signals were obtained in the frequency range of 210–250 MHz. The 59Co NMR frequency in fcc multi (single) domain particles is 217.4 (223) MHz. Possible contributions to the Co hyperfine field and changes to the Co magnetic moment in nanoclusters are discussed. An enhanced magnetic moment could be responsible for an increase in the hyperfine field leading to the observed NMR line at ∼243 MHz. Measurements of the transverse relaxation time (T2) as a function of temperature and field are reported.

Y(Co1-xAlx)2 is a weak itinerant ferromagnet for 0.12 < x < 0.20 with a maximum Curie point of about 25 K near x = 0.15. The pressure dependence of the Curie point for samples with 0.14 < x < 0.18 was measured in the temperature and pressure range 5–25 K and 0–8 kbar using a liquid filled lock cell. It was found that Tc was linear in pressure and extrapolated to OK at Pc = 9 ± 1 kbar for all samples. The volume expansion from YCo2 to x = 0.15 is equivalent to a chemical pressure of -40 kbar so ferromagnetism cannot occur at the lattice constant of YCo2.

The expanded lattice material Mn4N contains two types of Mn site. The 55Mn NMR at the Mn(I) site has been measured to 9 kbar at 77K. The Mn(I) NMR frequency, and the magnetisation, increased with pressure which is attributed to a decrease in the magnitude of the moment at the Mn(II) site.

Nuclear Magnetic Resonance is now such an enormous subject that only a simplified account of some selected topics can be given here. A semi-classical vector model of the nuclear magnetism will be sufficient for our purposes but a complete account of NMR requires the density matrix of quantum mechanics. The standard texts [1,2] provide the detailed derivation of the basic equations and more specialised topics are discussed in [3–11]. The emphasis of this chapter is on the NMR of metals, although the early sections are quite general. The high resolution NMR of liquids, a topic of enormous importance in chemistry and biology, is only mentioned in passing.

At room temperature the diffusion constant of hydrogen in many metals is comparable to that for ionic diffusion in water. The extensive theory of the NMR of liquids therefore provides a natural starting point for the discussion of hydrogen motion in metals but it should be recognized that there will be points of distinction between the two processes. For example, in a metal MHX the hydrogen moves between specific interstitial sites and therefore the motion of a given particle may be restricted as x increases because a neighbouring site is already occupied.

59Co and 55Mn NMR measurements have been made on a range of CoMn materials: dilute powder alloy, thin‐film alloys, and multilayers. Our results suggest that in dilute alloys isolated atoms of Mn couple both ferro and antiferromagnetically to the Co host, with the latter producing a reduction in the magnitude of the Co hyperfine field of 17%. This reduction of hyperfine field appears similar for both the fcc and hcp phases of Co. NMR on Co/Mn sputtered multilayers shows large changes in the hyperfine field distribution between a film with Mn layers of 10 A and Mn layers of 30 A.

The pressure dependence of the hyperfine field (HFF) of 197Au in iron has been measured at room temperature, with the result that ( delta lnHT/ delta P)T=-4.6+or-0.2)*10-4 kbar-1. This has been used to reduce measurements of the HFF, in the range 4.2K to room temperature, to constant volume. It is found that the temperature dependence of the HFF of Au at constant volume follows an exact T3/2 law over the whole range of temperature, whereas the HFF of iron deviates from the T3/2 law above 32K. It is suggested that the temperature dependence of the HFF of Au in iron may be governed by lattice dynamics.

The third-order elastic constants of cobalt have been determined from measurements of the hydrostatic and uniaxial pressure dependences of the velocities of ultrasonic waves propagated through single crystals. The hydrostatic pressure dependences of the second-order elastic constants have also been obtained. Comparison between these experimental results and predicted values show that, not unexpectedly, a simple Lennard-Jones model employed by previous workers for the lattice potential does not give a realistic assessment of the elastic behaviour of cobalt under pressure. The volume dependence of cobalt upon pressure, i.e. the compression, is estimated by extrapolation up to high pressure of the ultrasonic wave velocity data. The lack of acoustic-mode softening establishes that the h.c.p. to f.c.c. transition in cobalt is not associated with ferroelastic behaviour. The anharmonicity of the long-wavelength acoustic modes is discussed in terms of mode Gruneisen parameters obtained in the elastic con...

The 27Al zero-field NMR spectrum of GdAl2 at 4.2 K and 77 K has been shown to contain signals emanating from nuclei in both domains and all regions of domain walls. This interpretation is based on detailed measurements of quadrupole frequencies, and is supported by readings on a holmium-doped GdAl2 sample and measurements in an external magnetic field. The results are incompatible with a significant magnetic contribution to the electric field gradient at the AI sites unless such a contribution has its principal axis along the (111) axes in both domains and domain walls. Pressure measurements, giving a value of +(14+or-2) kbar-1 for dlnPt/dlnP where Pt is the quadrupole parameter and P the pressure, have been used to show that lattice expansion cannot explain the temperature dependence of the electric field gradient. Lattice vibrations, however, may provide an explanation.

The 27Al NMR in ferromagnetic iron was found to be a weaker function of temperature (at constant pressure) than the 57Fe NMR. The temperature dependence of the hyperfine coupling constant cannot therefore be attributed to a conduction electron contribution which decreases more rapidly than the magnetization.

The attenuation of longitudinal acoustic waves propagating along the (110) axis in single-crystal PdH0.64 has been investigated over the temperature range 80-300K and the frequency range 10-270 MHz. At a given frequency the attenuation exhibits a maximum as a function of temperature. The attenuation peaks are attributed to the stress-induced ordering of hydrogen atoms on the interstitial sites. These peaks occur when omega tau R=1, where omega /2 pi is the frequency of the ultrasonic wave and tau R is the relaxation time for stress-induced ordering. An Arrhenius plot of tau R-1 against inverse temperature gives an activation energy of 0.23+or-0.02 eV over the temperature range 190-240K, approximately the same as that deduced by other techniques for the hopping of hydrogen between interstitial sites. Surprisingly, the experimental value of tau R at any particular temperature is about an order of magnitude shorter than the mean time of stay of hydrogen on an interstitial site as determined by other experimental techniques. Possible reasons for this discrepancy are discussed.

Values of delta ln sigma / delta P, where sigma is the magnetisation per unit mass, for YFe2 (ZrFe2) of -8.4+or-0.4 (-6.3+or-0.3)*10-4 kbar-1 were obtained from forced magnetostriction measurements on polycrystalline samples at 4.2K in fields of up to 12 T. These values are shown to be in reasonable agreement with ab initio computer calculations of delta ln sigma / delta P but contradict earlier experimental work. The agreement between experiment and theory for delta ln sigma / delta P provides further evidence that YFe2 and ZrFe2 are ferrimagnetic.

The shape and width of the distribution of hyperfine fields in ferromagnetic iron and nickel sponge has been measured as a function of temperature using continuous wave NMR. The iron measurements are not in agreement with the theories of Friedel and de Gennes (1960) or of Suhl (1960). The apparent agreement for nickel may therefore be fortuitous.

An untuned NMR spectrometer that may be used in either a continuous wave or pulsed (spin-echo) mode is described and results shown of measurements made at frequencies up to 740 MHz. Operation up to 1 GHz or more should be possible.

The hyperfine field (Htau ) of V, Mn, Sb and Au in Fe and of Mn in Ni has been measured as a function of pressure to 15 kbar at room temperature and from 4.2K to room temperature as a function of temperature at atmospheric pressure. The magnitude of ( theta HT/ theta P) T for Mn in Fe is much larger than that of other 3d impurities, supporting the view that the well-known anomalous temperature dependence of the HFF of this impurity is due to a narrow bound state at the Fermi level. In the low-temperature limit only the HFF of Mn and Au in Fe exhibit a temperature dependence significantly different from that of the host magnetisation. The anomalous temperature dependence of the HFF of Sb in Fe near room temperature is removed by correcting to constant volume.

The hyperfine field (HT) of 89Y in YFe2 and YFe3 and of 57Fe in single-crystal and sintered polycrystalline YIG has been measured as a function of pressure to 15 bar at room temperature and for YIG also to 7 kbar at 77K. The value of ( delta ln HT/ delta P)T for YIG at 77K is zero to experimental accuracy but is positive at room temperature due to the pressure dependence of the Neel temperature. Little difference was found between the values for single-crystal and sintered YIG. The values of ( delta ln HT/ delta P)T for 89Y in YFe2 and YFe3 were found to be in general agreement with those to be expected if the Y hyperfine field (HFF) is produced by the polarisation of s like conduction electrons but a small contribution from the d band is probably required to explain the results in detail. The spin-wave stiffness coefficient of YIG was found to be (0.568+or-0.011) eV AA2 from an analysis of the temperature dependence of the HFF below 20K. This value is in good agreement with those found less directly by earlier HFF and magnetisation measurements and confirms that there is a large discrepancy with the value found by neutron scattering experiments. The reduced HFF at the two Y sites in YFe3 was found to have the same temperature dependence below 35K (T3/2 law) and in this region decreases far more slowly than was predicted from analysis of Mossbauer measurements made up to room temperature. Above 100K the difference in the reduced HFF at the two Y sites in YFe3 appeared to be proportional to T3/2 which, in view of the low-temperature result, must arise from a partial cancellation of higher order spin-wave terms.